Since blades and disks of a steam turbine used in a thermal power plant are exposed to a high temperature, these must have high properties such as creep rupture strength, creep rupture ductility, and oxidation resistance. In recent years, global environment protection, reduction of CO2 emission, and so on have been demanded, which also have posed a need for the thermal power plant to have higher efficiency.
The steam temperature of the steam turbine reaches 600 to 630° C., so that a ferritic heat-resistant 12Cr-steel has been used at present. To meet the need for still higher efficiency in the future, it has been studied to make the steam temperature as high as not lower than 700° C. However, the currently used ferritic heat-resistant 12Cr-steel lacks sufficient high-temperature strength at 700° C. Thus, it has been studied to use a austenitic γ′-precipitation-strengthening Ni-base superalloy excellent in high-temperature strength.
However, the Ni-base super alloy has some disadvantages of a high thermal expansion coefficient, low creep rupture ductility, tendencies of segregation, and a high price while having enough creep rupture strength.
Therefore, various studies have been made to solve these problems in order to make it possible to practically use the Ni-base superalloy in a 700° C.-class ultra super critical pressure thermal power plant.
In Patent publications 1 and 2, the present applicant has proposed a Ni-base alloy aiming at attaining satisfactory properties of a low thermal expansion coefficient, creep rupture strength, creep rupture ductility, and oxidation resistance in order to use it at a temperature of 650° C. In Non-patent publication 1, there is reported that various precipitation-strengthening Ni-base alloys were inspected about tendencies of macro segregation thereof, and that the Ni-base alloy proposed in Patent publications 1 and 2 is advantageous in producing relatively big size ingots because of those low critical values of occurrence of segregation.
Thus, the alloy proposed in Patent publication 1 or 2 has been noticed that it exhibits both of high temperature strength and hot workability when used for medium or small size forgings such as steam turbine blades and bolts and for big size products such as steam turbine rotors and boiler tubes.